This invention relates to a speed control means for an AC motor, and especially, for an AC commutator motor.
In the control of an AC motor to be used in any electrical driving means or machines such as machine tools or sewing machines, it is desired that the motor be rotated at a desired rotary speed with its output torque being kept constant irrespective of any variation in the loads applied thereto.
However, when a load is applied to a conventional AC motor which has been put in motion to be driven a desired rotary speed by the adjustment of a control resistance and the motor is rotating at the desired speed under the effect of the electric power supplied to the motor through the control resistance, the rotary speed of the motor is lowered when a load is applied and the desired rotary speed of the motor cannot be maintained without readjusting the control resistance thereof.
In addition, the output torque of the conventional AC motor is very low while the motor is rotating at a low rotary speed, so that it has almost been impossible to use a conventional AC motor for both high load and low speed operation.
In view of such disadvantages of the conventional electric motor, improvements have been made by the present invention.
That is, a main object of the present invention is to provide a speed control means for an AC motor by which the motor is rotated at a predetermined rotary speed with its output torque being kept substantially to constant irrespective of the variance of the loads.
Another object of the present invention is to provide a speed control means for an AC motor by which the motor can be caused to produce a desired output torque even at a very low rotary speed of the motor.
A further object of the present invention is to provide a speed control means for an AC motor by which the rotary speed of the motor can be adjusted from a very slow speed to a high speed obtained by directly connecting the motor to an alternating current source while keeping the output torque of the motor substantially constant.
Still another object of the present invention is to provide a speed control means as described above which is constructed from a very small number of circuit elements.
Another object of the present invention is to provide a speed control means for use in connection with a position setting means for an AC motor by which the motor can be caused to stop rotating at a desired rotary position.
Still another object of the present invention is to provide a speed control means for use in connection with a position setting means for an AC motor used for periodical mechanical operations wherein the position setting means functions to stop the rotation of the motor at a desired rotary position to assure safe mechanical operations.